When a gas sample is ionized in a mass spectrometer, an ionization method using thermal electrons, such as an electron ionization method and a chemical ionization method, is generally used. FIG. 6 and FIG. 7 are configuration diagrams of conventional general EI ion sources. Although an example case where positive ions are analyzed is described here, even in the case where negative ions are to be analyzed, a basic operation is the same except that the polarities of voltages are reversed.
A box-like ionization chamber 31 installed inside of a vacuum chamber (not illustrated) maintained at high vacuum has, formed therein: a sample introduction port 314 through which a sample gas is supplied; an ion emission port 311 from which ions are emitted; an electron introduction port 312 through which thermal electrons are introduced; and an electron discharge port 313 from which the thermal electrons are discharged. A filament 34 housed in a filament chamber 341 is arranged on the outer side of the electron introduction port 312. When a heating current If is supplied to the filament 34 from a heating current source (not illustrated), the temperature of the filament 34 rises, and thermal electrons are emitted from the surface of the filament 34. Meanwhile, a counter filament 35 housed in a filament chamber 351 is arranged as a trap electrode on the outer side of the electron discharge port 313. A voltage V1 of, for example, −70 [V] is applied to the filament 34. A voltage V2 of, for example, −71 [V], which is slightly lower than the voltage V1, is applied to the filament chamber 341. A positive voltage V4 of, for example, approximately +10 [V] is applied to the counter filament 35. Moreover, the ionization chamber 31 has a ground potential (0 [V]).
The thermal electrons produced from the filament 34 are accelerated by a potential difference (−71 [V]→0 [V]) between the filament chamber 341 and the ionization chamber 31, and are introduced into the ionization chamber 31 through the electron introduction port 312. The sample gas is introduced into the ionization chamber 31 from the sample introduction port 314. When a sample molecule M and a thermal electron e− contact each other in the ionization chamber 31, an electron emission of M+e−→M++2e− occurs. Consequently, a sample molecular ion or a sample atomic ion is produced. The electrons are attracted by the positive voltage V4 applied to the counter filament 35 to reach the counter filament 35, and a trap current Ib flows in the counter filament 35. The number of electrons trapped by the counter filament 35 depends on the number of electrons emitted from the filament 34. Hence, for example, a control circuit (not illustrated) controls the heating current If such that the trap current Ib has a predetermined value. This makes the amount of thermal electrons produced from the filament 34 substantially constant, so that stable ionization is achieved in the ionization chamber 31.
A pair of magnets 38 are installed on the outer sides of the filament 34 and the counter filament 35, and the pair of magnets 38 form a magnetic field inside and around the ionization chamber 31. Due to this magnetic field, the thermal electrons produced from the filament 34 and passing through the inside of the ionization chamber 31 toward the counter filament 35 fly on spirally whirling trajectories. Consequently, compared with the case where the electrons simply linearly fly, chances of contact between the electrons and the sample molecules increase, whereby the ionization efficiency can be enhanced.
In both the configurations illustrated in FIG. 6 and FIG. 7, sample-derived ions are produced in the ionization chamber 31 in such a manner as described above. The sample-derived ions thus produced are emitted to the outside of the ionization chamber 31 through the ion emission port 311 to be used for mass spectrometry. A mechanism for the ion emission is different between FIG. 6 and FIG. 7.
In the ion source illustrated in FIG. 6, a negative DC voltage V5 is applied to an extraction electrode 41 arranged on the outer side of the ion emission port 311. An electric field formed by a potential difference between the extraction electrode 41 and the ionization chamber 31 intrudes into the ionization chamber 31 through the ion emission port 311. Due to an action of this electric field, the ions produced in the ionization chamber 31 are extracted rightward in FIG. 6 (this operation is hereinafter referred to as an “extracting mode”), and are sent to a mass analyzer (not illustrated) such as a quadrupole mass filter.
In the ion source illustrated in FIG. 7, a repeller electrode 32 is arranged inside of the ionization chamber 31 and at a position opposed to the ion emission port 311, and a positive DC voltage V6 is applied to the repeller electrode 32. Due to an action of an electric field thus formed, the ions produced in the ionization chamber 31 are repelled rightward in FIG. 7 (this operation is hereinafter referred to as a “repelling mode”), are caused to pass through the ion emission port 311, and are sent to a mass analyzer (not illustrated). In some cases, both the ion repelling action of the repeller electrode 32 and the ion extracting action of the extraction electrode 41 are used.
In order to achieve high analysis sensitivity in the mass spectrometer, it is desirable to lead ions that are to be analyzed among the ions produced in the ionization chamber 31 to the mass analyzer with as low a loss as possible. Moreover, in order to maintain the reliability of a calibration curve used for quantitative determination, it is desirable that a decrease (change) in detection sensitivity when the apparatus is continuously used be as small as possible, that is, the stability of the sensitivity be high. However, as described below, it is difficult to achieve both high sensitivity and the stability of the sensitivity in the conventional ionization apparatuses.
In the case of the ion source adopting the repelling mode illustrated in FIG. 7, because the ions are repelled by the electric field formed by the voltage applied to the repeller electrode 32 arranged at a position away from the ion emission port 311, the potential gradient becomes gentler toward the ion emission port 311. Hence, ions produced in a region around a central part of the ionization chamber 31 receive sufficient energy to move toward the ion emission port 311 and are sent out from the ion emission port 311, whereas ions produced around the corners on the ion emission port 311 side in the ionization chamber 31 less easily pass through the ion emission port 311 and most of the ions disappear upon contacting a wall surface of the ionization chamber 31 around the ion emission port 311. Moreover, ions produced near the electron introduction port 312 and the electron discharge port 313 easily flow out through the electron introduction port 312 and the electron discharge port 313. As a result, in the repelling mode, only the ions produced in a relatively narrow region around the center of the ionization chamber 31 can be mainly used for mass spectrometry, and it is difficult to achieve high analysis sensitivity.
In the case of the ion source adopting the extracting mode illustrated in FIG. 6, the electric field that is formed outside of the ionization chamber 31 by the voltage applied to the extraction electrode 41 intrudes into the ionization chamber 31 through the ion emission port 311, and the ions are extracted by the extracting electric field thus formed. The extracting electric field that intrudes through the ion emission port 311 reaches even a region around the center of the ionization chamber 31, and hence the ions produced around the center of the ionization chamber 31 are favorably extracted from the ionization chamber 31. Moreover, the electric field around the ion emission port 311 near the extraction electrode 41 is strong, and hence, compared with the repelling mode described above, a larger amount of the ions produced around the corners on the ion emission port 311 side in the ionization chamber 31 can be extracted. Accordingly, compared with the repelling mode, the extracting mode can more efficiently send out the ions produced in the ionization chamber 31 from the ion emission port 311, and is more advantageous to enhancement of the analysis sensitivity.
Even in the extracting mode, the extracting electric field less easily reaches a region around the electron introduction port 312 and a region around the electron discharge port 313 in the ionization chamber 31, and hence the ions produced around these ports may flow out through the electron introduction port 312 and the electron discharge port 313. To solve this, in an ionization apparatus described in Patent Literature 1, ion leakage preventing electrodes each including an opening that allows passing of electrons are respectively arranged between the filament 34 and the electron introduction port 312 and between the electron discharge port 313 and the counter filament 35, and a predetermined voltage is applied to each ion leakage preventing electrode such that an electric field whose gradient becomes steeper for the ions from each of the electron introduction port 312 and the electron discharge port 313 toward the ion leakage preventing electrode is formed. Consequently, the ions that are about to flow to the outside from the electron introduction port 312 and the electron discharge port 313 are returned to the inside of the ionization chamber 31, whereby an ion loss can be suppressed. Hence, the ionization apparatus described in Patent Literature 1 is further advantageous to enhancement of the analysis sensitivity.
As described above, the extracting mode is more advantageous than the repelling mode in terms of achieving high sensitivity. However, according to studies made by the inventor of the present application, the extracting mode is more disadvantageous than the repelling mode in terms of achieving the stability of the sensitivity.
Specifically, as described above, in the extracting mode, the potential gradient for moving the ions is given by the intrusion of the electric field formed by the voltage applied to the extraction electrode 41 arranged outside of the ionization chamber 31, and hence the potential gradient around the center of the ionization chamber 31 is gentler than that of the electric field that is formed in the ionization chamber 31 in the repelling mode. Along with long-term use of the apparatus, a charge-up phenomenon may occur in the ionization chamber 31, which is made of a conductor, so that the state of the electric field formed in the ionization chamber 31 changes. As the potential gradient in the ionization chamber 31 is gentler, influences of such a change in electric field due to, so to speak, disturbance are larger. Hence, even if the analysis sensitivity of the extracting mode is initially higher than that of the repelling mode, a charge-up phenomenon associated with long-term use of the apparatus prevents the ions from being extracted along appropriate trajectories, so that the amount of ions that reach the mass analyzer significantly decreases, and the analysis sensitivity of the extracting mode becomes lower than that of the repelling mode.